The present invention relates to a centraliser for use in drilling applications. More particularly, the present invention relates to a centraliser having externally projecting rollers to minimise axial drag and internally projecting rollers to minimise rotational drag between the centraliser and an oil field tubular such as a casing, liner or drill string.
During the drilling of a well, centralisers may be employed to maintain the required alignment of the drill string. After a well has been drilled centralisers may be employed to centrally locate a liner or casing within the well.
When cementing a liner within a well it is desirable that the liner is able to rotate to ensure optimum cement distribution. To date centralisers have typically consisted of a tubular body having radially projecting fins with the liner simply located within the bore of the centraliser. Chemical lubricants have been added to drilling mud to minimise the friction between the centralisers and the liner. Chemical lubricants are effective once the liner has been rotated with respect to the centraliser. However, the initial torque required to rotate the liner may be very great and may be beyond the limit of the rig or may exceed the torque that can be borne by the liner connections.
It is also desirable to minimise the drag between a drilling string and the well bore when a centraliser is employed to reduce the torque required by the rig and minimise stress on components of the rig.
It is therefore an object of the present invention to provide a centraliser that reduces friction between the centraliser and drilling string, casing or liner or at least provide the public with a useful choice.
There is thus provided a centraliser comprising:
a generally tubular body;
a first group of rollers rotatably secured to the body at intervals about the body and positioned to project into the bore of the body to reduce rotational drag between the centraliser and an oil field tubular; and
a second group of rollers rotatably secured about the outer periphery of the body which project beyond the outer periphery of the body to reduce axial drag between the centraliser and the walls of a well.
The oil field tubular may be a drill string, liner or casing. The body of the centraliser may be of one part or two part construction.
The rollers are preferably cylindrical rollers tapered at their ends. The axles of the second group of rollers are preferably substantially parallel with the axis of the bore in the body. The axles of the first group of rollers are preferably substantially transverse to the axis of the bore of the body. The rollers are preferably designed so that should an axle break the roller will be contained within the centraliser body.